gnu: avr: Delay all cross compilation packages.

Partially addresses <https://issues.guix.gnu.org/65716>.

* gnu/packages/avr.scm: Add commentary comment.
(avr-binutils): Turn into this...
(make-avr-binutils): ... procedure.
(avr-gcc): Likewise, into...
(make-avr-gcc): ... this.  Adjust native-inputs accordingly.
(avr-toolchain): Likewise, into...
(make-avr-toolchain): ... this.
* gnu/packages/avr-xyz.scm (simavr) [propagated-inputs]: replace avr-toolchain
with a call to the 'make-avr-toolchain' procedure.

Series-changes: 2
- Use mlambda for procedures
- Fix ((make-avr-toolchain)) typo

gnu/packages/avr-xyz.scm

@@ -71,7 +71,7 @@
                            (string-append "PREFIX=" #$output)
                            (string-append "DESTDIR=" #$output))))
     (propagated-inputs
-     (list avr-toolchain))
+     (list (make-avr-toolchain)))
     (native-inputs
      (list autoconf
            which